1. Field of the Invention
This invention relates generally to the field of projection television receivers, and in particular to projection television receivers having screens providing significantly reduced color shift and/or significantly reduced cabinet depth.
2. Background Information
Color shift is defined as the change in the red/blue or green/blue ratio of a white image formed at the center of a projection screen by projected images from red, green and blue projection tubes, when viewed at different angles in the horizontal plane, by observations made at the peak brightness vertical viewing angle.
The color shift problem is caused by the need for at least three image projectors for respective images of different colors, for example, red, blue and green. A projection screen receives images from the at least three projectors on a first side and displays the images on a second side with controlled light dispersion of all the displayed images. One of the projectors, usually green and usually in the center of an array of projectors, has a first optical path in a substantially orthogonal orientation with the screen. At least two of the projectors, usually red and blue and usually positioned on opposite sides of the central green projector in the array, have respective optical paths converging toward the first optical path in a non orthogonal orientation defining angles of incidence. Color shift results from the non orthogonal relationship of the red and blue projectors, relative to the screen and to the green projector. As a result of the color shift, color tones may differ at every position on the screen. The condition in which the color tone difference is large is often referred to as poor white uniformity. The smaller the color shift, the better the white uniformity.
Color shift is denoted by a scale of numbers, in which lower numbers indicate less color shift and better white uniformity. In accordance with a common procedure, values for the red, green and blue luminance are measured at the screen center from a variety of horizontal viewing angles, typically from at least about xe2x88x9240xc2x0 to +40xc2x0, to as much as about xe2x88x9260xc2x0 to +60xc2x0, in 5xc2x0 or 10xc2x0 increments. The positive and negative angles represent horizontal viewing angles to the right and left of screen center, respectively. These measurements are taken at the peak vertical viewing angle. The red, green and blue data is normalized to unity at 0xc2x0. One or both of the following equations (I) and (II) are evaluated at each angle:                                           C            ⁡                          (              Θ              )                                =                      20            ·                                          log                10                            ⁡                              (                                                      red                    ⁡                                          (                      Θ                      )                                                                            blue                    ⁡                                          (                      Θ                      )                                                                      )                                                    ;                            (        I        )                                          C          ⁡                      (            Θ            )                          =                  20          ·                                    log              10                        ⁡                          (                                                green                  ⁡                                      (                    Θ                    )                                                                    blue                  ⁡                                      (                    Θ                    )                                                              )                                                          (        II        )            
where xcex8 is any angle within a range horizontal viewing angles, C(xcex8) is the color shift at angle xcex8, red(xcex8) is the red luminance level at angle xcex8, blue(xcex8) is the blue luminance level at angle xcex8 and green(xcex8) is the green luminance level at angle xcex8. The maximum of these values is the color shift of the screen.
In general, color shift-should be no larger than 5, nominally, on any commercially acceptable screen design. Other engineering and design constraints may sometimes require that the color shift be somewhat higher than 5, although such color shift performance is not desirable and usually results in a perceptibly inferior picture with poor white uniformity.
Screens for projection television receivers are generally manufactured by an extrusion process utilizing one or more patterned rollers to shape the surface of a thermoplastic sheet material. The configuration is generally an array of lenticular elements, also referred to as lenticules and lenslets. The lenticular elements may be formed on one or both sides of the same sheet material or on one side only of different sheets which can then be permanently combined as a laminated unit or otherwise mounted adjacent to one another so as to function as a laminated unit. In many designs, one of the surfaces of the screen is configured as a Fresnel lens to provide light diffusion. Prior art efforts to reduce color shift and improve white uniformity have focused exclusively on two aspects of the screen. One aspect is the shape and disposition of the lenticular elements. The other aspect is the extent to which the screen material, or portions thereof, are doped with light diffusing particles to control light diffusion. These efforts are exemplified by the following patent documents.
In U.S. Pat. Nos. 4,432,010 and 4,536,056, a projection screen includes a light-transmitting lenticular sheet having an input surface and an exit surface. The input surface is characterized by horizontally diffusing lenticular profiles having a ratio of a lenticulated depth Xv to a close-axis-curvature radius R1 (Xv/R1) which is within the range of 0.5 to 1.8. The profiles are elongated along the optical axis and form aspherical input lenticular lenses.
The use of a screen with a double sided lenticular lens is common. Such a screen has cylindrical entrance lenticular elements on an entrance surface of the screen, cylindrical lenticular elements formed on an exit surface of the screen and a light absorbing layer formed at the light non convergent part of the exit surface. The entrance and the exit lenticular elements each have the shape of a circle, ellipse or hyperbola represented by the following equation (III):                               Z          ⁡                      (            x            )                          =                              Cx            2                                1            +                                          "AutoLeftMatch"                                  [                                      1                    -                                                                  (                                                  K                          +                          1                                                )                                            ⁢                                              C                        2                                            ⁢                                              x                        2                                                                              ]                                "AutoRightMatch"                                            1                2                                                                        (        III        )            
wherein Cis a main curvature and K is a conic constant.
Alternatively, the lenslets have a curve to which a term with a higher order than 2nd order has been added.
In screens making use of such a double sided lenticular lens, it has been proposed to specify the position relationship between the entrance lens and exit lens, or the lenticular elements forming the lenses. It has been taught, for example in U.S. Pat. No. 4,443,814, to position the entrance lens and exit lens in such a way that the lens surface of one lens is present at the focal point of the other lens. It has also been taught, for example in JP 58-59436, that the eccentricity of the entrance lens be substantially equal to a reciprocal of the refractive index of the material constituting the lenticular lens. It has further been taught, for example in U.S. Pat. No. 4,502,755, to combine two sheets of double-sided lenticular lenses in such a way that the optic axis planes of the respective lenticular lenses are at right angles with respect to one another, and to form such double sided lenticular lenses in such a way that the entrance lens and exit lens at the periphery of one of the lenses are asymmetric with respect to the optic axis. It is also taught, in U.S. Pat. No. 4,953,948, that the position of light convergence only at the valley of an entrance lens should be offset toward the viewing side from the surface of an exit lens so that the tolerance for misalignment of optic axes and the difference in thickness can be made larger or the color shift can be made smaller.
In addition to the various proposals for decreasing the color shift or white non uniformity, other proposals for improving projection screen performance are directed to brightening pictures and ensuring appropriate visual fields in both the horizontal and vertical directions. A summary of many such proposals can be found in U.S. Pat. No. 5,196,960, which itself teaches a double sided lenticular lens sheet comprising an entrance lens layer having an entrance lens, and an exit lens layer having an exit lens whose lens surface is formed at the light convergent point of the entrance lens, or in the vicinity thereof, wherein the entrance lens layer and the exit lens layer are each formed of a substantially transparent thermoplastic resin and at least the exit layer contains light diffusing fine particles and wherein a difference exists in the light diffusion properties between the entrance lens layer and the exit lens layer. A plurality of entrance lenses comprise a cylindrical lens. The exit lens is formed of a plurality of exit lens layers, each having a lens surface at the light convergent point of each lens of the entrance lens layer, or in the vicinity thereof. A light absorbing layer is also formed at the light non convergent part of the exit lens layer. This screen design is said to provide sufficient horizontal visual field angle, decreased color shift and a brighter picture, as well as ease of manufacture by extrusion processes.
Several additional brightness problems occur due to the nature of projection systems. One of the common performance issues of a projection television design is the relative difference in brightness between the screen edges and the screen center under comparable degrees of illumination. Typically the picture at the corners is not as bright as at the center of the picture. The difference in relative brightness occurs partly because the optical path is shorter from the projectors to the center of the screen than from the projectors to the edges of the screen. The difference also occurs partly because the projectors are generally oriented toward the center of the screen, their beams typically converging at the center. The projectors thus illuminate the edges and corners both with less light intensity (due to distance) and less directly than at the center.
One method for dealing with edge brightness is to use a fresnel lens behind the diffuse or lenticular panel of the screen. The fresnel lens is a collimating lens having a focal length equal to the axial distance between the collimating lens and the exit lens pupils of the projectors. The object is to redirect light rays diverging from the projectors such that the rays along the projection axis from each projection tube emerge from the screen parallel to the axis.
A fresnel lens is subdivided into ridges that are progressively more inclined toward the edges of the lens, having a slope substantially equal to the slope of a solid collimating lens, the specific angles of the ridges being chosen such that refraction at the air/glass (or air/plastic) interfaces at the surface of the lens bend the rays in the required direction. In particular, rays diverging from the center axis of the screen are bent inwardly toward the center axis to emerge parallel to the center axis. This requires progressively greater refraction at the edges of the screen and no refraction at the center.
It is known in a conventional projection screen to increase the focal length of the fresnel ridges proceeding outwardly from the center of the picture. Off-axis light rays at the screen edges are bent beyond parallel to the center axis, and are directed somewhat inwardly toward the center axis. This makes the edges of the picture appear brighter provided the screen is viewed along the center axis, but is not helpful for viewing from other positions.
Another brightness variation problem can occur in projection televisions in which a fresnel is arranged to direct light in the direction of a user viewing from a point above the center of the screen, for example in a projection television having a relatively low cabinet. This is accomplished by offsetting the centerline of the fresnel upwardly relative to the center of the screen. Although this can improve relative brightness, especially at the corners, the top of the screen also appears generally brighter than the bottom of the screen.
Despite many years of aggressive developments in projection screen design, the improvements have been incremental, at best. Moreover, there has been no success in surpassing certain benchmarks. The angle of incidence defined by the geometric arrangement of the image projectors, referred to as angle xcex1 herein, has generally been limited to the range of greater than 0xc2x0 and less than or equal to about 10xc2x0 or 11xc2x0. The size of the image projectors and/or their optics, makes angles of xcex1 close to 0xc2x0 essentially impossible. In the range of the angles of xcex1 less than about 10xc2x0 or 11xc2x0, the best color shift performance which has been achieved is about 5, as determined in accordance with equations (I) and (II). In the range of the angles of a greater than about 10xc2x0 or 11xc2x0, the best color shift performance which has been achieved is not commercially acceptable. In fact, projection television receivers having angles of xcex1 greater than 10xc2x0 or 11xc2x0 are not known to have been marketed.
Small angles of a have a significant and undesirable consequence, namely a very large cabinet depth is needed to house a projection television receiver. The large depth is a direct result of the need to accommodate optical paths having small angles of incidence (xcex1). For a given size of the image projectors and optical elements, the angle of incidence can be reduced only by increasing the length of the optical path between the image projectors or their optics and the screen. Techniques for reducing the size of projection television cabinets generally rely on mirrors for folding long optical paths. The color shift success of such efforts is ultimately limited because there is a low limit to the range of possible angles of incidence.
Polaroid Corporation sells a photo polymer designated DMP-128(copyright), which Polaroid Corporation can manufacture as a three dimensional hologram, using proprietary processes. The holographic manufacturing process is described, in part, in U.S. Pat No. 5,576,853. Holographic photo polymers are generally useful for recording photographic images by splitting coherent light into an illumination beam and a reference beam. The illumination beam irradiates the subject. The reflected beam from the subject and the reference beam, which bypasses the subject, irradiate the photo polymer medium, which contains a developable light sensitive photographic composition. The light waves of the two beams interfere, that is, by constructive and destructive interference they produce a standing wave pattern of sinusoidal peaks which locally expose the photographic composition, and nulls which do not locally expose the composition. When the photographic medium is developed, a corresponding interference pattern is recorded in the medium. By illuminating the medium with a coherent reference beam, the image of the subject is reproduced and can be viewed over a range of apparent angles.
The recorded interference pattern of a hologram representing a typical photographic subject is complex because light from all the illuminated points on the subject interfere with the reference beam at all points on the hologram. It would be possible by recording the image of a blank xe2x80x9csubjectxe2x80x9d (effectively by interfering two reference beams), to make a blank hologram in which the interference pattern is more regular. In that case the interference pattern would resemble a diffraction grating but the pitch or resolution of the diffraction grating would be quite fine compared to the pitch of a projection screen having macro sized lenticular elements shaped to bend or refract light in a particular direction from rearward projection tubes.
A three dimensional holographic screen for a projection television was proposed -by Polaroid Corporation, as one of many suggestions made during efforts to establish a market for the DMP-128(copyright) photo polymer holographic product. The proposal was based on advantages which Polaroid Corporation expected in terms of higher brightness and resolution, lower manufacturing cost, lower weight, and resistance to the abrasion to which two-piece screens are subjected during shipping. Polaroid Corporation never proposed any particular holographic configuration for the volume holographic elements which might make up such a holographic projection television screen, and never even considered the problem of color shift in projection television screens of any type, holographic or otherwise.
Overall, despite years of intensive development to provide a projection television receiver having a screen with a color shift less than 5, even significantly less than 5, or having a color shift as low as 5 for angles of xcex1 even greater than 10xc2x0 or 11xc2x0, there have been no advances in solving the color shift problem other than incremental changes in the shapes and positions of lenticular elements and diffusers in conventional projection screens. Moreover, despite suggestions that three dimensional holograms might be useful for projection screens, although for reasons having nothing to do with color shift, there has been no effort to provide projection televisions with three dimensional holographic screens. A long felt need for a projection television receiver having significantly improved color shift performance, which can also be built into a significantly smaller cabinet, has remained unsatisfied.
A projection television receiver in accordance with the inventive arrangements taught herein provides such a significant improvement in color shift performance, measured in orders of magnitude, that a color shift of 2 or less can be achieved with projection television receivers having angles of incidence xcex1 in the range of less than 10xc2x0 or 11xc2x0. Moreover, the color shift performance is so significant that commercially acceptable projection television receivers having angles of incidence up to about 30xc2x0 can be provided, in much smaller cabinets. The color shift performance of such large xcex1 angle receivers is at least as good as conventional small xcex1 angle receivers, for example having a color shift of 5, and can be expected to approach or even reach values as low as about 2, as in the small xcex1 angle receivers.
These results are achieved by forsaking the extruded lens screen technology altogether. Instead, a projection television receiver in accordance with an inventive arrangement has a screen formed by a three dimensional hologram formed on a substrate, for example, a polyethylene film, such as Mylar(copyright).
Such a three dimensional holographic screen was originally developed for its expected advantages in terms of higher brightness and resolution, and lower manufacturing cost, lower weight and resistance to abrasion to which two-piece screens are subjected, for example during shipping. The discovery of the color shift performance of the three dimensional holographic screen came about when testing to determine if the optical properties of the three dimensional screen would be at least as good as a conventional screen. The color shift performance of the three dimensional holographic screen, as measured by equations (I) and (II), was so unexpectedly low as to be shocking. The barriers which limited prior art improvements to incremental steps had been eliminated altogether. Moreover, smaller cabinets with projection geometry characterized by larger a angles of incidence can now be developed.
A projection television having the unexpected properties associated with three dimensional holographic screens, and in accordance with the inventive arrangements taught herein, comprises: at least three image projectors for respective images of different colors; a projection screen formed by a three dimensional hologram disposed on a substrate, the screen receiving images from the projectors on a first side and displaying the images on a second side with controlled light dispersion of all the displayed images; one of the projectors having a first optical path in a substantially orthogonal orientation with the screen and at least two of the projectors having respective optical paths converging toward the first optical path in a non orthogonal orientation defining angles of incidence; and, the three dimensional hologram representing a three dimensional array of lenticular elements having a configuration effective for reducing color shift in the displayed images, the screen having a color shift less than or equal to approximately 5 for all the angles of incidence in a range greater than 0xc2x0 and less than or equal to approximately 30xc2x0, as determined by the maximum value obtained from at least one of the following expressions:                                           C            ⁡                          (              Θ              )                                =                      20            ·                                          log                10                            ⁡                              (                                                      red                    ⁡                                          (                      Θ                      )                                                                            blue                    ⁡                                          (                      Θ                      )                                                                      )                                                    ;                                          C          ⁡                      (            Θ            )                          =                  20          ·                                    log              10                        ⁡                          (                                                green                  ⁡                                      (                    Θ                    )                                                                    blue                  ⁡                                      (                    Θ                    )                                                              )                                          
where xcex8 is any angle within a range horizontal viewing angles, C(xcex8) is the color shift at angle xcex8, red(xcex8) is the red luminance level at angle xcex8, blue(xcex8) is the blue luminance level at angle xcex8 and green(xcex8) is the green luminance level at angle xcex8. The color shift of the screen can be expected to be less than 5, for example, less than or equal to approximately 4, 3 or even 2.
In terms of the known barrier at an angle of incidence of about 10xc2x0 or 11xc2x0, the color shift of the screen is less than or equal to approximately 2 for all the angles of incidence in a first subrange of angles of incidence greater than 0xc2x0 and less than or equal to approximately 10xc2x0; and, the color shift of the screen is less than or equal to approximately 5 for all the angles of incidence in a second subrange of angles of incidence greater than approximately 10xc2x0 and less than or equal to approximately 30xc2x0.
The screen further comprises a light transmissive reinforcing member, for example, of an acrylic material in a layer having a thickness in the range of approximately 2-4 mm. The substrate comprises a highly durable, transparent, water-repellent film, such as a polyethylene terephthalate resin film. The substrate can be a film having a thickness in the range of about 1-10 mils. A thickness of about 7 mils has been found to provide adequate support for the three dimensional hologram. The thickness of the film is not related to performance. The three dimensional hologram has a thickness in the range of not more than approximately 20 microns. The projection television may further comprise one or more mirrors between the image projectors and the screen.
According to an inventive aspect, a projection screen is specifically arranged to improve brightness and uniformity over a wide range of angles of incidence of the projection beams. This is accomplished using a holographic screen as described, having substantially higher gain proceeding toward the edges, backed by one or more linear fresnel panels having ridges that progressively vary in focal length from the center to the edges. The fresnel panel(s) of the screen also can have modestly different focal lengths between the bottom and top of the screen for optimizing screen, brightness and uniformity from the perspective of viewer located off the center axis of the screen.